Conventionally, there has been known a wet developing system in which the developing is performed by conducting an electrophoresis of toner particles on an electrostatic latent image on a surface of a photoconductor using a liquid developer which is formed by dispersing colorants, polymer particles and the like in a solvent of high electrical insulation. Further, according to the wet developing system, the toner particles contained in the solvent of the liquid developer are charged to a given polarity due to resin or a charge control agent which constitutes the toner particles and have a characteristic that the toner particles are easily dispersed in the solvent in a stable manner. Accordingly, the wet developing method, compared to a dry developing method, can perform the formation of image with high resolution using fine toner particles and, at the same time, the lowering of the local charge potentials due to leaking of charge can be suppressed and hence, the wet developing method is advantageous for the dry developing method in realizing the formation of image with high quality in a stable manner.
However, in performing the wet developing method, since a solvent of the liquid developer is required to have the high electrical insulation, a hydrocarbon-system solvent having high solubility such as isoparaffin is popularly used. Accordingly, such hydrocarbon-system solvent is brought into contact with a photosensitive layer for a long time and hence, a charge transport agent in the photosensitive layer is dissolved into the hydrocarbon-system solvent thus giving rise to a drawback that the sensitivity is lowered. Further, the binding region which forms the photosensitive layer swells due to the hydrocarbon-system solvent thus giving rise to drawbacks such as the softening of the photosensitive layer and the deterioration of durability attributed to the occurrence of cracks.
Accordingly, there has been proposed a technique which prevents the dissolution of a charge transport agent with the use of an organic photoconductor which forms an overcoat layer made of thermosetting resin on a surface thereof (see patent document 1, for example). However, the further formation of the overcoat layer gives rise to other drawbacks such as the remarkable deterioration of the sensitivity and the increase of a manufacturing cost.
Further, there has been proposed a technique which uses charge transport polymer for imparting a charge transport function to binding resin per se and decreases a content of a charge transport agent so as to increase the solvent resistance of the photosensitive layer (see patent document 2, for example). However, the molecular design of the charge transport polymer is not easy and hence, it is difficult to ensure the stable manufacture of the charge transport polymer thus giving rise to a drawback that the technique lacks the practicability. That is, the physical properties of the binding resin become irregular and hence, there have been drawbacks such as the irregular sensitivity characteristic of the photosensitive layer or the irregular dissolution amount of the charge transport polymer.
In view of such circumstances, inventors of the present invention have made an extensive study and have found out that when respective inorganic/organic values (I/O values) of an electron transport agent and a binding resin are set to values which fall within given ranges respectively or when a molecular weight of the electron transport agent and the inorganic/organic values (I/O values) of the binding resin are set to values which fall within given ranges, due to the interaction between these materials, the dispersibility and the stability of a hole transport agent are enhanced and, at the same time, the liquid developer can be manufactured in a stable manner. Further, as a result, the inventors have also found out that when the liquid developer is used in an image forming apparatus of a wet developing method, the liquid developer exhibits the favorable solvent resistance, wherein the charge transport agent (hole transport agent or electron transport agent) is hardly dissolved in a hydrocarbon-system solvent and a favorable image is obtainable.
That is, it is an object of the present invention to provide a wet-developing electrophotographic photoconductor which can be manufactured stably by making use of particular physical property indexes of an electron transport agent and a binding resin and possesses the excellent durability and the excellent solvent resistance and to a wet-developing image forming device which uses such a wet-developing electrophotographic photoconductor.    [Patent document 1] JP10-221875A    [Patent document 2] JP2003-57856A